Electrochemical sensors are widely used to determine electroactive chemical species in liquid, gas and vapor phases. Such electrochemical sensors or cells can be conveniently classified as galvanic when operated to produce electrical energy and electrolytic when operated at a constant potential via consumption of electrical energy from an external source. Many electrochemical sensors can be operated in either a galvanic or an electrolytic mode. A comprehensive discussion of electrochemical gas sensors is provided in a paper by Cao, Z. and Stetter, J. R., entitled "Amperometric Gas Sensors," the disclosure of which is incorporated herein by reference.
In a typical electrochemical sensor, the chemical entity to be measured (the "analyte") typically diffuses from the test environment into the sensor housing through a porous or permeable membrane (through which the analyte is mobile, but through which the electrolyte is not mobile) to a working electrode (sometimes called a sensing electrode) wherein the analyte chemically reacts. A complementary chemical reaction occurs at a second electrode in the sensor housing known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte at the working and counter electrodes.
In general, the electrodes of an electrochemical sensor provide a surface at which an oxidation or a reduction reaction occurs (that is, an electrochemically active surface) to provide a mechanism whereby the ionic conduction of an electrolyte solution in contact with the electrodes is coupled with the electron conduction of each electrode to provide a complete circuit for a current. By definition, the electrode at which an oxidation occurs is the anode, while the electrode at which the "complimentary" reduction occurs is the cathode.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte over the concentration range of interest.
In addition to a working electrode and a counter electrode, an electrolytic electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte and carry the lowest possible current to maintain a constant potential.
As discussed above, the electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The primary functions of the electrolyte are: (1) to efficiently carry the ionic current; (2) to solubilize the analyte; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. The primary criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
Acids, such as sulfuric acid (H.sub.2 SO.sub.4) or phosphoric acid (H.sub.3 PO.sub.4) satisfy the above criteria very well and are, therefore, often used as electrolytes in electrochemical sensors. Given the use of acidic electrolytes, the components of electrochemical cells, including the housing therefor, must be fabricated from corrosion-resistant materials. Certain basic electrolyte solutions also satisfy the above criteria.
Because of corrosion and the corrosion currents caused by the interaction of acids and metals, metallic housings have never been used with acidic electrolytes. Indeed, applicants are aware of only one sensor having a metallic housing. However, this sensor incorporates a basic electrolyte, not an acidic electrolyte. See, for example, U.S. Pat. No. 4,132,616. The housings for electrochemical sensors containing acidic electrolytes are fabricated from acid-resistant, polymeric materials, such as certain injection molded plastics. Millions of such plastic housing electrochemical sensors have been fabricated over the last several decades. Unfortunately, injection molded plastic housings for electrochemical sensors suffer from a number of significant drawbacks. For example, plastic housings are relatively expensive to manufacture. Moreover, plastic housings are somewhat gas permeable and often result in the leakage of gases (for example, oxygen) through areas of the housing other than the sample inlet port. Such leakage can cause erratic performance of the electrochemical sensor as well as high background or base currents. Increasing the thickness of the plastic reduces the permeability but increases the size and cost of the sensor. Additionally, the relatively low thermal conductivity of plastics can lead to significant temperature gradients across the sensor, resulting in unpredictable performance. It is very desirable to develop electrochemical sensors that do not suffer from these and other drawbacks.